Swiss does this.
On my regular ZRH-AMS route the exit rows are all unavailable yet when you get on the plane they are vacant.
I know the obvious answer is the capability question but I think its more to do with capacity (empty flight, seats unreserved, fuller flight, seats held back.
@ Pedalbin, I suspect the young able couple checked in online ahead of you or had a better class of ticket, my experience of booking seats and having them reserved for you depends on your class/type of ticket. However this is a guess based on limited group travel as I generally fly on my own and don't care.